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Date:   Wed, 28 Dec 2022 19:24:38 +0000
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Subject: [V4 PATCH 4/4] KVM: selftests: x86: Invoke kvm hypercall as per host cpu
From:   Vishal Annapurve <vannapurve@google.com>
To:     x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org
Cc:     pbonzini@redhat.com, shuah@kernel.org, bgardon@google.com,
        seanjc@google.com, oupton@google.com, peterx@redhat.com,
        vkuznets@redhat.com, dmatlack@google.com,
        Vishal Annapurve <vannapurve@google.com>
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Invoke vmcall/vmmcall instructions from kvm_hypercall as per host CPU
type. CVMs and current kvm_hyerpcall callers need to execute hypercall
as per the cpu type to avoid KVM having to emulate the instruction
anyways.

CVMs need to avoid KVM emulation as the guest code is not updatable
from KVM. Guest code region can be marked un-mondifiable from KVM
without CVMs as well, so in general it's safer to avoid KVM emulation
for vmcall/vmmcall instructions.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vishal Annapurve <vannapurve@google.com>
---
 tools/testing/selftests/kvm/lib/x86_64/processor.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
index 18f0608743d1..cc0b9c17fa91 100644
--- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
+++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
@@ -1154,9 +1154,15 @@ uint64_t kvm_hypercall(uint64_t nr, uint64_t a0, uint64_t a1, uint64_t a2,
 {
 	uint64_t r;
 
-	asm volatile("vmcall"
+	asm volatile("test %[use_vmmcall], %[use_vmmcall]\n\t"
+		     "jnz 1f\n\t"
+		     "vmcall\n\t"
+		     "jmp 2f\n\t"
+		     "1: vmmcall\n\t"
+		     "2:"
 		     : "=a"(r)
-		     : "a"(nr), "b"(a0), "c"(a1), "d"(a2), "S"(a3));
+		     : "a"(nr), "b"(a0), "c"(a1), "d"(a2), "S"(a3),
+		       [use_vmmcall] "r" (is_host_cpu_amd()));
 	return r;
 }
 
-- 
2.39.0.314.g84b9a713c41-goog